90. Family Coronaviridae – SARS Virus

I. General Characteristics

• Coronaviridae are enveloped, positive-sense, single-stranded RNA viruses.
• Genome is non-segmented, largest among RNA viruses (~30 kb).
• Helical nucleocapsid, replicates in cytoplasm.
• Named for the “crown-like” (corona) appearance due to surface spike (S) glycoproteins.
• Transmission is mainly through respiratory droplets, direct contact, and aerosolized secretions.
• Reservoir: Bats (natural host), with intermediate hosts like civet cats in zoonotic SARS-CoV-1.

III. SARS Virus (Severe Acute Respiratory Syndrome Coronavirus – SARS-CoV-1)

A. Virulence Factors

• Spike (S) protein:
• Binds to ACE2 receptor (mainly in lungs, intestine, kidney).

• Triggers fusion and entry → defines tissue tropism and host range.
• Envelope (E) protein:
• Facilitates virus budding and release; contributes to inflammation.
• Accessory proteins:
• Interfere with host interferon signaling, delaying immune response.
• Nucleocapsid (N):
• Enhances viral RNA synthesis; modulates host cell stress responses.
• Downregulation of ACE2:
• Leads to imbalance in the renin-angiotensin system → lung injury.

B. Pathogenesis

• Initial entry via respiratory epithelium → virus uses ACE2 receptor to enter cells.
• Replication in epithelial cells causes cytopathic effects, apoptosis, and inflammation.
• Systemic spread can involve:
• Lungs: diffuse alveolar damage → ARDS (Acute Respiratory Distress Syndrome)
• GI tract: diarrhea (virus found in stool)
• Immune system: cytokine storm in severe cases → multiorgan failure.

C. Clinical Features

• Incubation period: 2–10 days.
• Symptoms:
• High fever (>38°C), dry cough, malaise
• Dyspnea (within ~1 week), diarrhea (20–25%)
• No coryza (runny nose is rare, unlike common cold)

• Severe complications:
• ARDS, respiratory failure, need for ventilation
• Secondary bacterial pneumonia
• High mortality (~10%, higher in elderly)

D. Diagnosis

• RT-PCR: gold standard (detects viral RNA in respiratory samples).
• Serology: detection of antibodies (IgM/IgG) for retrospective diagnosis.
• Chest imaging:
• Ground-glass opacities, bilateral infiltrates (similar to COVID-19).
• Viral culture: possible but high biohazard risk.

E. Treatment

• No proven antiviral; treatment is supportive:
• Oxygen, mechanical ventilation
• Corticosteroids used cautiously
• Investigational drugs: interferons, lopinavir/ritonavir (limited success)

F. Prevention and Control

• No licensed vaccine for SARS-CoV-1.
• Control measures:

• Isolation, quarantine of contacts
• Strict infection control in hospitals
• Travel screening and surveillance
• Lessons from SARS guided later COVID-19 responses.